Sealable, biaxially oriented polyester films are known in the art.
GB-A 1 465 973 describes a coextruded, double-layered polyester film, one layer of which consists of copolyesters containing isophthalic and terephthalic acid and the other layer of which consists of polyethylene terephthalate. Due to the absence of filler the film cannot reliably be manufactured (the film cannot be wound) and is only limited suitable for further processing.
EP-A-0 035 835 describes a coextruded, sealable polyester film, to the sealing layer of which contains particles with an average particle size exceeding the thickness of the sealing layer in order to improve the winding and processing characteristics. These particulate additives form protrusions at the surface, which prevent any unwanted blocking and sticking of the film to rollers or guides. No further details in terms of the use of antiblocking agents are given about the other, non-sealable layer of the film. It remains unclear whether or not this layer contains any antiblocking agents. No statements about the sealing temperature range are made in the document. Seal seam strength is measured at 140° C. and is within the range from 63 to 120 N/m (corresponding to 0.97 to 1.8 N/15 mm film width).
EP-A-0 432 886 describes a coextruded multi-layered polyester film which has one surface, equipped with a sealable layer, and a second surface, equipped with an acrylate layer. Here too, the sealable cover layer may consist of copolyesters containing isophthalic acid or terephthalic acid. Coating the reverse side improves the processing characteristics. No statements about the sealing temperature range are made in the document. Seal seam strength is measured at 140° C. For a sealing layer with a thickness of 11 μm a seal seam strength of 761.5 N/m (corresponding to 11.4 N/15 mm film width) is specified. The disadvantage of coating the reverse side with an acrylate layer is that this side can no longer seal against the sealable cover layer. Thus the film is only limited useable.
EP-A-0 515 096 describes a coextruded, multi-layered, sealable polyester film which contains an additional additive on the sealable layer. The additive may, as an example, contain inorganic particles and is preferably applied to the film during its production in the form of an aqueous coating. This allows the film to maintain its good sealing properties and provides a good processability. The reverse side contains only very few particles which get into this layer mainly via the regranulate. Even in this document no statements about the sealing temperature range are made. Seal seam strength is measured at 140° C. and is greater than 200 N/m (corresponding to 3 N/15 mm film width). For a sealing layer with a thickness of 3 mm a seal seam strength of 275 N/m is specified.
WO 98/06575 describes a coextruded, multi-layered polyester film containing a sealable cover layer and a non-sealable base layer. The base layer may consist of one or more layers with one of these layers being in contact with the sealable layer. The other (outer) layer then forms the second, non-sealable cover layer. Here too the sealable cover layer may consist of copolyesters containing isophthalic acid and terephthalic acid. However, these copolyesters do not contain any antiblocking particles. Apart from that, the film contains 0.1 to 10% by weight of an UV-absorber which is added to the base layer. The base layer of this film is provided with customary antiblocking agents. The film may also have a mat look. The film has good sealing properties, it does, however, not have the desired processing characteristics and has deficiencies in its optical properties (gloss and opacity).
Also known in the prior art are mat, milky-appearing, biaxially-oriented polyester films.
DE-A 23 53 347 describes a process for the production of a mono or multilayered, milky polyester film, wherein a mixture of particles from a linear polyester with 3 to 27% by weight of a homopolymer or a copolymer made of ethylene or propylene is made, the mixture is extruded to form a film, the film is chilled and biaxially oriented by way of stretching it in orthogonal directions, and wherein the film is heat-set. The disadvantage of this procedure is that the regrind resulting from the production (mainly a mixture of polyester raw material and ethylene or propylene copolymer) can no longer be reused since otherwise the film turns yellow. This makes the process uneconomical and the colored film produced with regrind could not succeed in the market. Increasing the concentration of copolymer in the polyester generally causes the film to lose its milky character and it turns white with a high opacity.
It was the object of the present invention to provide a one-sided mat, sealable, biaxially oriented polyester film without the disadvantages of the previously mentioned films according to the prior art and with outstanding optical properties (high degree of matness with simultaneous good transparency), a very good sealability and an improved processability. Moreover a safe and trouble-free processability even on high-speed processing machines should be guaranteed. During the film production it must additionally be guaranteed that up to 60% by weight of the waste material, based on the total weight of the film, resulting from the film production can be reused for the film production, without having a noticeable adverse effect on the physical and optical properties of the film manufactured in that manner.